> most people would find the idea of picking up a weapon and killing random innocent people to be completely abhorrent. 

Killing random guilty people, on the other hand, is completely legitimised in the USA. Self-defence advocates talk about it all the time. So all that has to happen is for someone to manufacture a justification for the victim(s) deserving it and it becomes legitimate in their eyes.

That's how lynching used to happen; entire communities would turn out to murder some people simply because they were black.